Adopt old test code in `tests_old` to new test code in `tests`

The current `tests` dir contains new tests written from scratch. The old tests (with tensorly) where moved to `tests_old` for the sake of keeping the code until the tests are ported. 

When porting the tests we should try to minimize duplicate code, i.e. make use of `@pytest.mark.parametrize` and fixtures. 